Telephone exchange system



May 24, 1932. M. a. M DAVITT 1,859,404

TELEPHONE EXCHANGE SYSTEM Filed Dec. 19, 1930 2 Sheets-Sheet 1 lNVENTOR M. 8. Mr: DA V/ 77' ATTORNEY May 24,1932. B, MCDAWTT 1,859,404

TELEPHONE EXCHANGE SYSTEM Filed Dec. 19, 1950 2 sheets-sheet 2 FIGZ - lNl/ENTOR M..B. MC DA l l7'7' A T TORNEV Patented May 24, 1932 UNITED sT'rA ES PATENT orricr.

MARCELLUS B.- MCDAVITT, OF NEW YORK, N. Y.,..ASSI G1 TOB 530 AMERICAN TELEPHONE AND TELEGRAPH COMPANY, A CORPORATION GTE NEW YORK TELEPHONE EXCHANGE SYSTEM Application filed Dece mb'er 19.1930. Serial No. 503,601

This invention relates to telephone systems and moreparticularly to automatic and semiautomatic telephonesystems in which switching. mechanisms are directly responsive to incoming impulses. I

The object of this inventionis to eliminate the possibility of prematurely, releasing a step-by-step switch upon the permanent opening. of theimpulse circuit at the point where the impulsesareoriginated.

A feature of this invention is an impulse repeater so arranged that the incoming impulse circuit may be permanently opened without causing the opening of the outgoing impulse circuit.

Another. feature of this invention is an intermediate-dialing trunk circuit in a stepby-st'ep oliice. arranged-sot-hatthe answering and. dialing cords, associated. therewith on any particular call, may be disconnected in either sequence without causing the release of the automatic'switches through which con:- nection with the. called line is established.

The drawings which consist ofFigures 1 and 2, represent schematically a portion of a telephone system in which this invention is embodied.

Fig. 1 shows a subscribers station A, a cordcircuit B ina manual oiiice, an incoming trunk circuit IT inf an automatic ofiice and an order tone'circuit OTO';

Fig. 2 shows the remainder oftheincomingztrunk" circuit IT,. a cord circuit CG at a position of the A switchboardini the automatic office, an operators telephone circuit OT, the associated dialing circuitDC an incomingselect'or switch IS, a connector switch C and 21 called subscribers station D.

In the's-ystem shown in. the drawings, calls from subscribers in the manual oiiice to subscribers in the automatic office are completed over interoiiice trunks which terminate at positions of the special A switchboard in the automatic oflice. These trunks are'operated on a straight-forward basis, a call being answered atthe automatic o iice by inserting the plug of an answeringcord in'the answering jack associated with the lighted lamp oi an incoming trunk; Upon receipt of an order tone from the automatic office, the manual ofii'ce-ope'rator passes the number of the i called station to the operator at the automatic oflice The operator in the automatic oificeth'eninserts the plug of the calling cord in a dialing ack associated with the incoming trunk and dials the called number, thereby automatically extendingthe connection to the called line; The incoming trunk circuit in the automatic oliice is known as an intermediate dialing trunk circuit since the operator in the automatic oiiice' does not supervise the connection but disconnects both the answering and calling cords from the answering and dialing jacks as soon as the called number has been dialed, the connection being under the supervision of the operator in the manual ofiice. i

The subscribers station A in the manual office is equipped with a common battery type subset and the cord circuit B is of the usual type provided in common battery manuallyoperated .oliices. .As already indicatedthe outgoing interofiice trunks areof the straightforward ty-pe. ITis shown and hereinafter described in its entirey but the associated incoming selector IS andthe connector G are represented by their multiple brushes and one set of terminals, the remainder of these'eircuits being omitted. It is of course understood-that in larger systemsone or more additional selectors-may be required in extending-a connection from the incoming selector IS to a con"- nector' switch. The selector and connector switches are of the well known Strowger type and'reference may be had to pages 53= to 65 in clusive of the second edition of Automatic Telephony by Smith and Campbellfor a com plete description of the operation of these switches. ,The' position equipment at the special A switchboard of the automatic ofl'ice is not shown in its entirety, only so much of'the cord circuit GO, dial circuit DC, and

order tone circuit OTG being shown as is necessary tor a clear and complete description of the-features of theinventionr. The order tone circuit .OTG diflers somewhat" from the. usual arrangement since it isdesigned for use withintermediate dialing-.trunka,

The operationof the circuits shown in the The incoming trunkv circuit peating coil 5 of cord circuit B, the tip conductor, of the calling cord '8, the tip spring of jack 9, the tip conductor of trunk 10,.the

upper left-hand winding of the repeating coil 11 of trunk circuit IT, through both windings of relay 12, the lower left-hand winding of repeating coil 11, the ring conductor of trunk 10, the ring spring of jack 9,

I relay 66, the sleeve conductors closes a circuit from battery winding springsof relay 30, the ring 62, the lower the ring conductor of the calling cord 8, the winding of supervisory relay 6, and through the lower right-hand winding of the repeating coil 5 toground. The resistanceof this circuit is so high that the supervisory relay 6 in the cord circuit 13 cannot operate and the supervisory lamp 7remains lighted until the called party answers. Relay 12 closes a circuit for operating the slow-to-release relay 32; this circuit is traced from ground through the front contact of relay 12, the

outer upper back contact of relay 34, and

through the winding of relay 32 to battery. tela'y 32 closes a circuit for lighting an answering lamp which is associated with an answering jack 61 located at a position of the A switchboard in theautomatic office; this circuit is traced from battery, through the front contact of relay 32, the back contact of'relay 31, the outer lower back contact of relay 14, and through lamp 60 to ground.

Assuming that an operator answers the call byinserting the plug of the answering cord CC into the answering jack 61, a circuit is closed from battery through the winding of of the answering cord 62 and jack 61, and through the winding of relay 31 to ground; relays 66 and 31 both operate in this circuit. Relay 31 extinguishes the answering lamp 60 and through the of relay 30, the front contact of re- My 31, the back contact of the continuity spring of-jack 61, the ring conductor of the answering cord left-hand winding of repeating coil 67, and through thelower winding of relay 63 to ground. Relay 30 operates in this circuit, but relay 63 does not operate until a circuit is closed through both of its windings as hereinafter described. Relay 30 opens its own operating circuit but it looks underthe control-of relay 31 to the ground at the front,

contact of its continuity spring. Relay 30 connects the windlng of relay 16 to the pickup conductor 20 of the order tone circuit OTC. If the tone circuit is idle, relay 16 operates in a circuit which is traced from battery, through its winding and the normally closed contact of its continuity springs, the outer lower front contact of relay 30, pick-. up conductor 20,'the back contact of relay 22, V to ground at the normally closed contact of the continuity springs of relay 28. Relay 16 locks through the front contact of its continuity spring to ground at the inner lower front contact of relay 30. Relay 16 closes a circuit from battery through the winding of relay 14, the inner upper back contact of relay 34, the'normally closed contact of the continuity springs of relay 14, the outer upper front contact of relay 16, over the start conductor 19 to the order tone circuit OTC, and through the windings of start relay 21 to ground. Relay 21 operates but relay'14 is marginal and it does not operate in series with both windings of relay 21. Relay 21 closes an obvious circuit for operating relay 22. Relay 22 connects the secondary wind- 7 ing of the tone transformer 23-to' the income ing trunk circult IT so that the order tone is transmitted through repeating. coil' 11 to the operatorin the manual oifice; this connection is traced from the upper end of the secondary winding of transformer 23, through the outer upper front contact of relay 22, through condenser 25, over conductor 17 to the trunk circuit IT, throughthe inner upper front contact of relay 16, the inner lower back contact of relay 14, the lower right-hand winding of repeating coil 11, winding of supervisory relay 13 in parallel with condenser 15, the upper right-hand winding of repeating coil 11, the inner upper back contact of relay 14, the middle upper front contact of relay 16, over conductor 18, through the lower back contact of relay 28, the back contact of relay 26, and through the lower front contact of relay 22 to the other end of thesecondarywinding of tone transformer 23. 7 Relay 22 also closes an obvious circuit for operating relay 26, and the operation of relay 26 opens the aforementioned circuit through the secondary winding of tone transformer 23. The operation of relay 26 alsocloses a circuit for operating relay 27; and relay 27 again closes the circuit through the secondary winding of tone transformer 23, this circuit now being traced'through the lower front contacts of both relays 26 and 27 instead of through the back contact of relay 26. Relay 27 also closes a circuit for operating relay 28, and relay'28 again opens the circuit'for transmitting the order tone to the conductors oftrunk circuit IT. =Withrelay 28 operated, the ground at the back'contact of its continuity springs cannot be reconnected to the pick-up conductor 20 until all of the relays in the order tone circuit have tone circuit result; thefirstbeginswhen relay 22 operates and ends when relay 26 operates, and the second begins when relay 27 operates and ends when relay 28 operates. 'lVith relay 28 operated, the lower winding of" start relay 21 is short-circuited so that the marginal relay 14 in the trunk circuit IT now operates. Relay 14 locks through the front contact of its continuity spring to the ground at the up-.

per front contact of relay 30. Relay 14 shortcircuits condenser 29 so that the connections from the right-hand windings of repeating coil 11 are now extended through the outer back contacts of relay 38 and the tip and ring springs of the answering jacks 61'to' the windings of relay 63 in the cord circuit CC. The operation of relay 63 connects resistance 64 in parallel with the answering cord super- Visory lamp 65 to extinguish this lamp.

In response to the double order tone which was transmitted through the windings of re pea-ting coil 11 over the conductors of trunk 10, the operator in the manual office passes the number of the called station D to the operator at the A switchboard in the automatic office, it being assumed that the talking key 71 was operated. at the time that the plug of the answering cord 62 was inserted in the answering ack 61. The operator in the automatic office then inserts the plug of the calling cord 78 into the dialing jack 79 associated with the trunk circuit IT, thereby closing a circuit for operating relay 74 in the cord circuit and relay 40 in the trunk circuit. Relay 40 closes a circuit for operating relay 37; and relay 37 connects ground to the sleeve conductor 53 leading to the associated incoming selector IS. With ground connected to conductor 53, relays 42 and 43 are operated and the lower winding of re lay 44 is energized, but relay 44 is not operated by the energization of the lower winding alone. Relay 42 connects ground to conductor 33 so as to hold relay -14 operated, after the operator in the automatic: office has disconnected, until the connection is-released by th'e'operator in the manual office. Relay 43 closes abridge across the conductors 51 and 52 leading to the incoming selector switch IS this bridge may be traced from conductor 51 through the continuity spring and back contact of relay 38, the outer upper front contact of relay 43, retard coil 39, the outer back contact of relay 35, and through the inner lower back contact of relay 34 to conductor 52. Relay 43 also closes a circuit through the upper winding of relay 44 and this relay now operates thereby opening the operating circuit for relay 43; but relay 43 does not release since a locking circuit was closed through its inner upper front contacts and the back contact of relay 47 to the ground on conductor 53.

The aforementioned operation of relay 74, when the plug of the calling cord 78 was inserted in jack. 79, closes a circuitz'for. oper atingrelay'7 6. Relay: 7 6 closes, a: circuit: for lighting the calling. cord supervisory-lamp 70. Relay 7 6- also closes a. circuit; for opera ating the impulse receiving relay 48 of the trunk. circuit IT this circuit is traced from battery through the lower winding of; relay 48,: the upper router back" contact I of. relay 50, the ring conductors of j aclr 79 and cord-78,

through. the inner lowerfront contact of rethe normally closed. contact of the upper 7 continuitys'pri'ngs of relay: 75, the upper out:-

' er front contact of relay 76,..the tip conductors of cord 7 8 and jack 79, theinner' upper back contact of relay 50,; and through; the. upper winding of relay 48toground. Relay 68' is a polarized relay and doesnot operate since the current through its winding is'not. in the operating direction. When relay 48 0 er.- ates it connects the windingof relay 4 I to the condenser 49 and relay 47. is, therefore, temporarily operated: during the flow of current to chargethe condenser 49.

When theoperator. at the automaticoffice moves the; dial80 01'f-normal,.relayr81 in" the dial circuit DC operates; ,Relay 81 closes a circuitforloperating relay in the cord circuit CC; this circuit is traced from battery through the windingwo-f. rela 75, the inner upper front contact of relay 6 ,the back contact of relay'73, the lower contacts of key 71, and through the lower front contact of relay 81 toground at the backcontact: of relay 82; Relay 75 closes; a locking circuit for itself which is traced through' its inner lower front contact, the back; contact of-re;

lay 85 andthrough the winding of relay 82 to ground. Relay 82 is operated inthis circuit thereby lighting thepilot lampf83. Relay 75 transfers the tip and ring conductors of? the answerin cordfrom the-windingsx-of repeating coil 6 to the dial circuit DC, so that relay 48 is now held operated by a bridge which is tracedfrom the front contact of the upper continuitysprings ofnrelay 7 5, the ininer upper front contact of relay 81, and through the interrupter contacts of. the dial to the front contact" of the lower con tinuity springs of relay 75. Relay 75 also closes a circuit for operatingrelay 73. Re.-

lay 73 looks under control o-f relay 74 and opens the operating circuit for relay 75 so as to prevent the reoperation of relay75 after it has once released. a

As thedial returns to normal itinterrupts the circuit of relay 48 so that relay48is al ternately released and reoperated'in-response toth'e impulses created by the dialingof each digit of the numberofthe; called station'D. The release of-rclay 48 upon receipt of an back-contact of relay 48 for discharging condenser 49. When relay 48 reoperates, relay 47 temporarily reoperated by the current which flows to chargecondenser 49; and. the

operation of relay 47 causes the release of relay 43, thereby opening the bridge across conductors 51 and 52; Relay 43also opens the Circuit through the upper winding of re-' lay 44 so that relay 44 is; also released. The release of relay 44 again closes the operating circuit for relay 43 and'the reoperation of relay again closes the bridge acrosscom ductors 51 and 52. Relay 43 also causes the reoperation of relay 44; and. with relay 44 operated, relay'43: is held operated by its locking contact imder control of relay 47. Each release'and reoperation of relay 48 causes relays 43, 44and 47 to repeat their cycle of operations and thereby transmit inipulses over conductors 51 and" 52 to the succeeding switches. The condenser 45 and resistance .46 control the releasing time of relay 44, being designed to prevent the reoperation of relay 43'until the bridge across the conductors 51 and 52 has been held open for an interval suflicient to insure proper operation of the switch to which impulses are being transmitted. A

In response to the impulses thus transmittedover conductors 51 and 52, the selector switch IS and the connector switch C operate to extend the connection over the line 100 to the called station D in the usual manner. As soon as all of the digits of the called number have been dialed, the operator disconnect-s the cord CC from jacks 61 and 7 9, the release of the connection being under the control of the-operator at the manual office. If the plug of the calling cord is withdrawn before the plug of the answering cord is withdrawn, relays 74 and 40 release. The release of relay 4O closes av circuit for operating relay 36 from battery through the winding of relay 36, "ou ter lower back contact of relay 34, inner upper back contact of relay 40, and through the inner lower front contact of relay 42 to ground. Relay 36 holds relay 37 operated to prevent the release of relay 42. Relay 48 releases when the calling cord is withdrawn from jack 79, but the bridge across conductors 51 and 52 is not opened since relay 47 is not operated and consequent- 1y relay 43 does not'r'elease. When the answering cord is also disconnected, relays 66 and'31 release therebycausing the operation of relay 38 in a circuit fromground through the winding of relay 38, the lower front contact ofrelay 42, the back contact of relay 31,

and through'the front contact of relay 32- to battery. Relay 38 closes a holding circuit "for relay 37 and extends the talking conductors of trunk IT to conductors 51 and 52,

so that the call is now entirely under the control of the operator in the manual ofiice.

The

aforementioned operation of relay 38 also closes the circuit for operating relay 50; this circuit may be traced from battery; through the winding of relay 50, the middle'upper.

48 and 40. The marginal relay .14 is now 1 held operated by the ground connected to conductor 33 at the outer upper front contact of relay 42. All of the relays in the cordcirvcuit CG and dial circuit DC are released when the cord is disconnected from the. jacks 61 and 79, and these circuits may now be used for completing other connections.

' If the plug of the answering cord is withdrawn from jack 61, before the plug of the calling cord is withdrawn from the jack, 79, the release of-relay 31 closes the circuit for operating relay 38 as above described and relay 38 locks under control of relay-32. Re.- lay 38 disconnects the talking conductors of the trunk IT from theanswering cord and extends these, conductors to the selector switch 18. Relay 38 also closes a holding circuit for relay 37. When the calling cord is disconnected relay 4O releases, thereby operating relays 36 and 50. Relay 50 disconnects the windings of relays 48 and40 from the dialing jack 7 9'; and-relay 36 closes another holding circuit for relay 37.' v

It is apparent, therefore, that relay 43 holds coil 39 bridged across conductors '51 and 52 until the right-hand windings of repeating coil 11 in serieswith the winding of supervisory relay 13 are connected across these conductors; so that the selector switch IS and connector switchC are not released until the operator in the manual oflice disconnects the calling cord 8 from jack 9, as hereinafter described. T

In case the operator at the A switchboard in the automatic oflice wishes tocorrect a dialing error, the plug of the dialing cord is removed from jack 79 and then reinserted without disconnecting the answering cord. When the dialing cord is disconnected, relays 48, 74 and 40 release. Therelease of relay 48 is ineffective to open the bridge across conductors 51' and 52 but the release of relay 40 causes the operation of relay 36. The release of relay 74 causes the release of relays 73 and 76; and the release of relay 76 causes the calling cord in jack 7 9, relay 4O reoperates. The reoperation of relay 40 causes the release'ofi relay 36; but'relay 36:is:sl0w in! releasing-"southat a circuit :is temporarily closed for-operating relay 35"; this circuit-may be 7 traced: from: battery through the Winding of 4 relay; 35 the lower'front contactof relay36;

theinnenupper front contact 0115 relay 450, andi throughthe innen lower" front contact oflf relay. 42'. tel groundi Relay opens: the

bridgeacrossconductors51 and52 and closes the middle upper front contact of jrelay' 4.0.-

Relay4'1' closes a circuit for operatingrelay 50 and also closes a circuit fer-holding relay' 4:0 operated after relay- 50' has disconnectedthe=winding of relay 4:0' from the-sleeve con-- du'ctor of='jack-' 7 91 When relay: 361 releases,

relays-3T and 3,5 are released; rel'ay also being slow, in releasing. Relay 37- disconnects ground f-rom'sleeve conductor 53 therehycausingthe release ofrelay'sLQ; l3 and44i F When relay 35 releases, rela y 37 is-again operated to-thegr-ound' at the-outerupper frontcontact of relay-- 40;" and the reoperationof relay 37 again: connects ground to conductor 53 therebycausing' the reoperation' of relays A 42, 43- 'and- 4A; as hereinbefore described.

With relay 35 released andrelay- 4:3 reoper= ated, the retard" coi139 is again connected across conductors 51- and 52 but the bridge was heldopen 'a* sufficient length of time to causet-herelease ofalliof the switches'which had been operated in. response: toimpulses transmitted over these conductors. The aforementioned release-ofrelay 35 also causes the release of relay ll andf the release ofrelay ll causes the release of relay 50. With relay 5'0 released, thewinding ofrelay l0'is again connected to the sleeve conductor of jack 79. so thatrelay 40 is noW held operated in'series With-relayMof the cord circuit CC.

lhe relays in the cord circuit GCand dial 45" circuit- DGare-now operated in the manner hereinbefore described to close the dialing circuit from the dial to the windings of re1ay'4t8 inthe' trunk circuit IT and the operator proceeds to dial the, correct number,

When the called subscriber answers,,rel'ay 13' is operated bythe usual reversal of cur;

rent supplied) from the connector switch C. With relay 713' operated; the high resistance Winding ofrelay 12 i'sShOrt-Circuited so as. to increasethe current flowing over trunlil'O' and thereby cause the operation of the super: visory relay 6 in the cord circuit B in the manual oflice. The operation of'the, relay 6 a extinguishesthe lamp 7 in the usualimanner 60" to indicate that the called subscriber, has answered;

When the connection is releasedgthe operator-ih the manualfoffice disconnects the cords 3 and 8 from the ,jfacks 2' and 91 thereby cause ing the release of relay 1'2. Relay 12 causes the-.releaseiof relay 32; and when relay '32: releases 1 it causes therelease of relay 38- andi theoperation:of re1ay 344 The-releaseof re lay -3-8-"opens: the :circuitthrouglt the Winding off relay. 50 and disconnects conductors: 51;; and 52"from= the winding of the repeating coil lili With relay-3'44 operated; the releaseof relay 38 is inefl'ectiWe-toeonnect the lietardi coilg 39: across: conductors: 51* and 59/ so that theselectou switclr 1S2 and' connector; which; C aizeneleased: and restoredito normal in the usual. manner; The opeua'tion oi? re my 3%: causes the release of relays 1 4 and 363 The release. ol relay: 36 is: effective to cause the release-.oii relay 371;: and the release of 'nelay 37Jcausesti1eneleasea-ofi relay-s42; 48 ands-lei, All" of the i circuits have now; been= restored! to normal and are 1 ready for: use on anothen connectionu Whatzis claimed is: j p R 1 In. combination; a; relay: fer: receivingr impulses, a! circuit means for causing theoperatihn oi: saidimpulse receiving. 'relay'and the closingoh said: circuit and means-- for openingand; closing-said; circuit in; response toreach; impulse recei ve'd by said; relayand for holding said; circuit closed when: said:

relay: releases; a I

'2;.In; combination; a: finsticircuit a second circuit, arr impulse receivingrelay included! in-i said first circuit, means for"'causin-grthe operation at sai-de relay; and thev closing oi said; SGSOIldi circuit, means efleoti've inf response tioie'ach;impulsevreceitzed by. said relay for; repeating said impulse-ntoz'said second cincuit. and; for holilinggsaid: second circuit.- closed: when said first circuit is i opened.

3. In; combination, a first. circuit; aesecondi circuit; an impulse. receiving relay: included i'ui said; first cincuit,-. means-1 tor causing the initiatopenatdomofsairl relayuandthe closing: of saidiseqonidcincuityanjdimeans foropening andzclosingrsaidlsecond circuitinresponseto BEIClIziIHPUlSe receiyieds by said relay, said; last mentioned: means being ineflective; t0: open. saldacincuit when said; relay) releases:and does; not'reoperateb o 7 In aitelephonee system ai circuit include my; an-limpulseisender andan) im ulsemeeeiw ingrelayaattnunkg,aniiilductance;.meansieflec tive to cause the operatiom'ofi saidi relay-and: the. bridging; of. said; inductance; across the conductors; of; said: trunk; means for succes sizvzely; openingrand; closing-said bridge lIIiIG spnnse to impulsesrreceiuedtby said relay and for. holding said bridge closed when said circuit: is: permanently opened 51 In a telephone system; an operaeo'rs position, an' impulse sender at said position-,1 atrunk means for operatifvel'y associating" said senderwith saidtrunk, an impulse re ceiving relay'indivi'dual to said trunk for re ceiying impulseslcreatediby the operation. of,

its

let

permanently connected, means for closing a pulsing circuit over said conductors, means for causing the operation of said impulse receiving relay, means for repeating the impulses received by said impulse receiving relay to said pulsing circuit and for holding said pulsing circuit closed after said sender has been disconnected.

6. In a telephone system, an operators position and telephone circuit, a cord circuit at said position, a line terminating at an answering jack, an impulse sender, an automatic switch, conductors extendingfrom said line to said switch, a second jack individual to said f conductors, means responsive to the insertion of the plug of the answering cord' into said answeringrjack-for' extending said line to said telephone circuit, an impulse receiving relay, means responsive to the insertion of the plug of the calling cord into said second jack for associating said impulse sender with said impulse receiving relay, a bridge connected across said. conductors in consequence of said insertion of the plug of the callingcord, meanszforsuccessively opening and closing said bridge in response to im-- pulses received by said relay and for holding said bridge closed when said relay releases.

7.'In a telephone system, an operators position-and telephone circuit, a cord circuit at said position, a line terminating'at an answering jack, an impulse sender, an automatic switch, conductors extending from said line to said switch, a second jack individual to said conductors, means responsive to the insertion of' the plug of the answering cord into said answering jack for extending said line to said'tele'phone circuit, an impulse receiving relay, means responsive to the inser,

tion of the plug. of the calling cord into said second jack for associating said impulse sender with said impulse receiving relay, a bridge connected across said conductors in consequence of the insertion of the plug of the calling cord into said second jack, means for successively opening and closing said bridge in response to impulses received by said relay and for holding said bridge closedwhen said relay releases irrespective ofwhich of said plugs isfirst removed from theassoci'ated jack. 7

8. In a telephone system, an operators position and telephone circuit, a cord circuit at said position, a line terminating at an answering jack, an impulse sender, an automatic switch, conductors extending from said line to said switch, a second jack individual to said conductors, means responsive to the insertionof the plug of the answering cord into said answering jacli for extending said. line to said telephone circuit, animpulse receiving relay, means responsive to the insertionof the plug of the calling cord into said second jack forassociating said impulse sender with said impulse receiving relay, a

of the removal and reinsertion consequence of the insertion of the plug of r the calling cord into said-second jack, means for successively opening and closing said bridge in response to impulses received by said relay and for holding said bridge closed when said relay releases irrespective of which of said plugs is first removed from the associated jack, and Vmeans for openingrsaid bridge to release said switch in consequence of the plug of the calling cord. l

9. In a telephone; system, an impulse sender, an impulse receiving relay, a circuitfor operating said impulse, receiving. relay, a trunk, acoil, means for closing said circuit and'for causing said coil to be bridged across the conductors of said trunk, an impulse circuit including the interrupter contacts of said sender and the windings of said im-.

pulse receiving relay, means responsive to the alternate releaseia'nd reoperatlon of said mpulse receiving relay for opening and closing said bridge and for holding said bridge closed when said impulse receiving relay releases and does not reoperate. we

10. In a telephone system, an impulse sender, an impulse receiving relay, a first circuit for energizing said impulse relay, a second circuit including the interrupter contacts of said sender forenergizing said impulse relay, a trunk, a bridge for connection across the conductors of said trunk, means for: closing saidfirst circuit and for causing the'closure of said bridge, means for opening and closing said bridge in response to impulses received by said impulse relay from said sender and for holding said bridge closed when'said impulse relay releases and does not reoperate, and means for temporarily opening said bridge when said first relay reoperates. r I p 11. In a telephone system, a line having incoming conductors and outgoing conductors, an impulse sender, means for associating said sender with said line, means for operatively 12. In a telephone system, a line having incoming conductors and outgoing conduca tors, an impulse sender, means for associat ing said sender with said line, means for op-s eratively associating said sender with said outgoing conductors, a br dge for connection across said outgoingconductors, means for closing said bridge priorto the operative as sociation of said sender and outgoing line, means responsive to impulses created by the operation of said sender for opening and closing said bridge and for holding said bridge closed after said sender is disconnected from said line, means for connecting said incoming and outgoing conductors, and means effective prior to the interconnection of said incoming and outgoing conductors for temporarily opening said bridge.

13. In a telephone system, a line having incoming conductors terminating in an answering jack and outgoing conductors terminating in a dialing jack, an operators position and telephone circuit, said position circuit including a dial, a cord circuit having an answering cord and plug for connecting said position and telephone circuit with said incoming conductors and having a calling cord and plug for connecting said position circuit with said outgoing conductors, a bridge for connection across said outgoing conductors, means responsive to the insertion of said calling plug into said dialing jack for closing said bridge, means responsive to impulses created by the operation of said dial for opening and closing said bridge and for holding said bridge closed when said calling plug is re moved from said dialing jack, and means for transferring said outgoing conductors from said bridge to said incoming conductors.

In testimony whereof, I have signed m name to this specification this 18th day of December, 1930.

MARCELLUS B. McDAVITT. 

